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Forces "felt" by pilot, G-meter, inclinometer--are they the aerodynamic forces generated by the aircraft, or the sum of weight+centrifugal force?
Consider a pilot sitting at the CG of an aircraft, with instruments also located at the CG of the aircraft.  (Feel free to consider other cases as a bonus, but the core of the question is aimed at this simple case.)
Consider the apparent force "felt" by the pilot, and measured by a traditional-style panel-mounted G-meter, when the aircraft is upside-down at the top of a loop.  Should this force considered to simply be an expression of the wing's lift vector, plus any other aerodynamic force components that the aircraft is generating?  Does gravity make a contribution to this force?  Does "centrifugal force" make a contribution to this force?  Is the force "felt" by the pilot, and the G-meter, due to the combined effects of gravity and "centrifugal force"?  Can more than one of these things be true at the same time?
Consider the apparent force "felt" by the pilot, and determining the deflection of the inclinometer (slip-skid ball), during a coordinated turn, or during an uncoordinated turn.  Is this force simply an expression of the net aerodynamic force generated by the aircraft?  Does gravity make a contribution to this force?  Does "centrifugal force" make a contribution to this force?  Is the force "felt" by the pilot, and the inclinometer, due to the combined effects of gravity and "centrifugal force"?  Can more than one of these things be true at the same time?
Feel free to substitute "acceleration" for "force" in your reading of the question, if you feel it makes it a better question.
Basically this question is seeking to reconcile the view that a pilot only "feels" the aerodynamic force generated by an aircraft, and the view that gravity and / or centrifugal force also play a role in the forces "felt" in flight.  Can these two views be reconciled?  If so, how?  Or is only one of these two views correct?
A thought problem might that better inform the question -- consider an astronaut in orbit.  Is he or she feeling both centrifugal force and gravity?  Or neither?  Or is this purely a manner of convention, i.e. either answer is accurate?

Since comments are not for extended discussion on ASE, I've created a chat room for this topic -- https://chat.stackexchange.com/rooms/107177/lets-talk-about-gravity-can-you-feel-it

We would have a great opportunity to put these concepts into ACTUAL PRACTICE if we voted to re-open the question https://aviation.stackexchange.com/questions/77528/what-is-missing-from-this-diagram-of-the-forces-in-slips-and-skids

The answer lies in a breakdown of the term aerodynamic.  It may be obvious, but I will say it anyway:  "Aero" is the Latin word for air.
So is it possible for the human body to perceive an aerodynamic force?  Absolutely.  Leaning into a gale force wind, sticking your hand out the car window, skydiving, even the sensation of a light breeze across your face are all examples of feeling an aerodynamic force.
Absent direct contact with outside air though, other perceived forces in a moving vehicle are not, (by definition) aerodynamic.  When you are in an airplane, (or car, or train...) you are not directly feeling aerodynamic force, other than from the vents.  You are effectively sealed off and shielded from the outside forces acting on the airframe.
That's why you can walk down the aisle of an airliner to use the lavatory without fighting against 500 knots of drag.
Gravity is the primary force perceived by the human body that we are most familiar with.  In a moving object, an additional force may be perceived when the direction of travel is changed.  What you are feeling is your momentum being restrained.  This is called centrifugal force, and anybody who has ridden in a car has felt it going around a corner.
Many would argue that centrifugal force isn't a "real" force.  From an engineering standpoint I accept this, and am not going to argue that point here.  But it is easily perceived, demonstratable, and offers the best explanation of G force to the lay person.
And it is real enough:  It is real enough to keep that water inside the bucket you swing over your head.  It is real enough to keep your body pinned to the wall when the floor drops on the spinning Gravitron ride at the county fair.  It is real enough to be misperceived as (vertical) gravity in a shallow turn when visual cues are absent.  And it is real enough to cause the fighter or acrobatic pilot to black out at 7 Gs if countermeasures are not used.
If it enhances your understanding of "real" forces to negate centrifugal force for computation purposes then please disregard my explanation.  But if you are trying to relate what pilots feel in the aircraft to more common everyday experiences, then maybe this answer will offer some simple clarity.
In summary, the aerodynamic forces the aircraft generates produce the centrifugal force that, (added to earth's gravity) the body experiences.

The specific problem with this answer is the last sentence. The net centrifugal force is not the result of only the aerodynamic forces. For why this is relevant see chat comments   https://chat.stackexchange.com/transcript/message/54138356#54138356 and https://chat.stackexchange.com/transcript/message/54138386#54138386 .

The body feels earth's gravity, AND any lateral Gs produced by the aircraft turning.  You do not subtract out gravity.  If there is something factually incorrect, or if you have a better way of phrasing that last sentence then please offer a concise suggestion.  But I am not going to let this drag into another protracted circular discussion with you.  My whole intent with this answer was to steer away from overthinking something that is fundamentally simple and relatable to other things non-pilots are able to observe.  In other words, no math.

If the last sentence of the answer were corrected to be true (i.e. centrifugal force is produced by aerodynamic effects PLUS GRAVITY) , it would show that saying the body feels centrifugal force plus gravity is inherently circular and while not false, is really just equivalent to saying the body feels only the aerodynamic effects.  Explained in more detail in chat permalinks above.

From a practical standpoint the following are equivalent:  "the aerodynamic forces the aircraft generates produce the centrifugal force that, (added to earth's gravity) the body experiences" = "centrifugal force is produced by aerodynamic effects PLUS GRAVITY".  When the floor drops on the Gravitron ride the horizontal centrifugal force pins you to the wall, but you still feel vertical gravity from the earth.

... and is was just the kind of nit-noid circular discussion I was hoping to avoid.  Please, let's stop saying essentially the same thing in subtly different ways.

@MichaelHall-- since comments are not for extended discussion on ASE, I've created a chat room for this topic-- https://chat.stackexchange.com/rooms/107177/lets-talk-about-gravity-can-you-feel-it

Can more than one of these things be true at the same time?

Yes. It is only a matter of the point of view, or, more precisely, the reference system.
Consider a trivial case first: The aircraft sits on the ground, engine off. The pilot will feel his weight pressing into the seat even in the absence of aerodynamic forces. What he feels is gravity, and the force is the product of his mass and gravitational acceleration.
Now do the same with the aircraft flying in level flight. The forces acting on the pilot are still the same, so it is again gravity what he feels. Only that now the aircraft is not supported by its landing gear and the ground, but by wing lift (and the ground again).
Next step: The pilot flies a dive, followed by a 2g pull-up, meaning that the g-meter reads 2 gs. Again we look at the moment when the aircraft is level, but now it flies on a curved flight path. This adds a centrifugal load of 1g to the 1g of gravity and the pilot feels twice the weight force that he felt before. The nitpickers will now say this is not weight, because weight is the product of mass and gravitational acceleration, but our pilot couldn't care less at the moment. It hurts in his buttocks, that's what counts.
And neither the g-meter nor the pilot could tell gravity apart from centrifugal force – they all feel the same and only the sum of all is felt as a single sensation.
Now switch reference systems and look at the flow around the wing. Speed and angle of attack combine to produce a lift force of twice the weight the aircraft had in level flight. This is pure aerodynamics, no gravity or centrifugal load involved. Strictly speaking, the aerodynamic force on the wing is even a bit higher in order to compensate for the higher tail downforce resulting from trim to overcome pitch damping. Looking at the full aircraft as one object, its total lift force is now twice as high as in level flight.
This force gets transmitted to the pilot seat and the instrument panel and is felt there as a resistance against the acceleration. Depending on your point of view, either the pilot is accelerated into his seat or the seat pushes against the pilot. Actio equals reactio.
Next switch: The same curved flight path, but now at the moment when the aircraft is inverted. Now centrifugal and gravitational acceleration cancel each other, the g-meter reads zero and the pilot feels weightless. In order to do so, the pilots needs to push the stick forward and reduce wing lift to about zero, or wing lift would pull the aircraft down. At this moment we have no aerodynamic contribution to the vertical forces felt by both the pilot and the g-meter.
Now the pilot pushes the stick even more forward so his flight path becomes straight and level again, only with the aircraft in an inverted position. The g-meter now reads -1 g, the force of the harness pressing on the pilot's shoulders is again his weight times gravitational acceleration, but what allows the harness to support the pilot is the lift of the wing the harness is attached to. Change your reference system, and it is again aerodynamics that the pilot feels. Let him wiggle the stick and the change in aerodynamics is immediately felt on his shoulders.

@quietflyer: No problem, I'm open for any suggestions. My only concern is the ever growing length of my answers …

All above is based on Newton.. .But Newton was wrong. Using Newton to explain aircraft motion only makes the issue unnecessarily  complicated. The only forces we "feel" in an aircraft are the forces generated by the engines, and by the aerodynamic pressure of the air on the surface of the airframe. Everything else is engineering self-abuse done to explain away the math because we are trying to measure everything in a non-inertial (accelerated) frame of reference.

If you believe you can actually feel the force of gravity, then please explain why the feeling goes away when you push forward on the stick and unload? Do you believe that gravity actually has been turned off by the forward motion of the stick? Is it not so much simpler that what you "felt" was simply the lift on the wings, (not Gravity). And when it goes away, you no loner feel it.

@Charles Bretana, what we feel as gravity is actually resistance to gravity.  I.e., when something is stopping us.  A body in free fall is certainly experiencing gravity, but it feels like weightlessness.

@Charles Bretana yes, the earth and the moon rotate around the sun, which rotates around our galaxy, which is moving away from other galaxies.... But everything on earth acts in the earth frame of reference.  The effect of gravity is like acceleration, but, in the earth frame, at 1G, we are not accelerating relative to earth.  A full explanation of "gravity" awaits us all, but in an airplane it is simply balanced with vectors of acceleration.

@CharlesBretana Don't split hairs – as I said, you feel the sum of all accelerations, all at once. In the absence of inertial accelerations, only gravity remains. Of course you feel gravity then. Or, as Michael Hall observes, its consequences.

@PeterKampf and others -- Since comments are not for extended discussion on ASE, I've created a chat room for this topic -- https://chat.stackexchange.com/rooms/107177/lets-talk-about-gravity-can-you-feel-it

Robert, consider. In one G level flight, what we feel pushing on our butt is Gravity? If that is true why are we not feeling the forces of the lift on the wings? Is it not the case that an aerodynamic force is pushing on the bottom of the aircraft? Are we feeling that? Or not? If not, why not? If we are, and as you say, we Are Also feeling the force of gravity, then why don't we feel twice as heavy? How do you explain this?

Also, If you can "feel" gravity, then why are the astronauts in the ISS weightless? They are, indisputably, in orbit because of the "force" of gravity... Yet they feel no force! In fact, if they turn on a rocket pointed at the earth, with enough thrust to maintain a straight path tangent to their orbit, THEN, they would feel a force. Yet you would say they are "feeling" the force of gravity, and not the force of the rocket? That is, at minimum, confusing and obfuscating, and at worst, patently ridiculous.

And you don't "feel" acceleration, you "feel" forces... Again, what about astronauts in orbit? they are ostensibly (in the earth's frame of reference), undergoing an acceleration, yet they are weightless! They feel nothing. In order to explain this, you either need to adopt other obfuscations, (like they did in 1600s with the "Cycles in Cycles" they used to explain planetary motion), or abandon your wrong assumptions about the earth-bound frame of reference being special, representative of "reality" in a way that other frames of reference with different accelerations are not.

@CharlesBretana: Why is it so hard for you to accept that the sum of all accelerations is felt? Gravity is part of that. And to say that weight but not acceleration is felt is really nitpicking about terms. Force is mass times acceleration. If you don't like my answer, feel free to downvote. But stop that silly harassment!

@PeterKämpf, But Peter, the statement you make  "the sum of all accelerations is felt" is just wrong. Accelerations are not "felt".  Forces are felt. What you fail to understand is that accelerations are RELATIVE to the frame of reference you measure them in, just as velocity, and position are.  Read http://en.wikipedia.org/wiki/Inertial_frame_of_reference Try to understand this before you accuse others of not understanding.

or just think about it.. .If you measure  acceleration in a frame of reference that is itself ACCELERATING, do you not see that the acceleration of the frame of reference itself will change the answer you get? Just like measuring Velocity in a moving Frame of reference will change the measurement of an object's velocity!

so the equation F=ma is only valid within an inertial frame of reference (where there is NO acceleration - where a G-Meter fixed to the frame would read ZERO.) In any other frame that is accelerating relative to an inertial frame Fictitious forces such as Gravity, Centrifugal force, etc. MUST be included to make the math work out. But you cannot "feel" them, they are FICTITIOUS!

It's the acceleration of the object laterally due to a change from a straight path to a curved one.  You're just along for the ride.  If you are spinning a ball around on a string on a vertical plane, I think you could look at it as the lift vector being the string.  As the ball passes the bottom of the arc, you have centrifugal force (the ball being forced to move in an arc) plus gravity applying tension on the string; the string, or the lifting force, is subjected to the weight of the ball plus centrifugal force imposed by moving in an arcing path.  Both the plane and you as pilot feel your weight plus the magnification of your weight due to the acceleration created my moving in an arc, when at the bottom of the loop, and feel (and the plane's structure subjected to), typically 3+ Gs down (Gravity + acceleration off a straight path adding to it).
And at the top, it's centrifugal force MINUS gravity.  In the spinning string/ball, if the rotation is too slow, centrifugal force becomes less than gravity, the string goes slack and the ball falls.  In the airplane, if the rotation is too slow, or you don't hold the correct arc with the elevator, you go zero G, the wings aren't making any lift at all (slack string) and may fall out of the top of the loop if you try to make the wing fly inverted to hold the loop's arc (this is how pilots get into inverted spins), or at minimum your loop becomes D shaped and the judges mark you down or the airshow crowd boos.
So what the pilot feels is gravity, plus or minus the acceleration created by moving in a non-linear path.  At the bottom of a loop, apparent gravity is below, but magnified. At the top of the loop, apparent gravity is above, because lateral acceleration imposed by the arcing movement is more than gravity (at the top of a loop you normally feel maybe one quarter to half a G because the centrifugal acceleration is about 1.2-1.5G total; if you barely make it over the top and end up at only 1G of centrigual acceleration, it's fully cancelled out by gravity and you will feel weightless and your're right on the edge of falling out - best not push especially if you don't have an inverted fuel and oil system).
When skidding, it's just the same acceleration applied laterally, which sums with gravity to provide an new "apparent" gravity.  Spin the ball on the string horizontally, but slowly so that the string's movement describes a cone.  The angle of the string is the "apparent" gravity felt by the ball, and is also what your perceive as "down" when skidding in an airplane.  Bank the plane to an angle appropriate to the rate of change in direction, and now the apparent gravity is aligned with the airplane's vertical axis and the glass of water on the instrument panel sits level to the panel as if you were stationary and everything is right with the world.

"at the top of a loop you normally feel maybe one quarter to half a G" – well, I always try to round the loop at the top instead of flying a flight path looking like a standing egg. When I did this in an open cockpit Stearman once, the flight instructor in the back seat desperately cried "pull, pull!" because he forgot to close his pockets. I don't mind the negative gs at the top of the loop.

If you're doing loops in a clip wing Cub with the fuel tank and wire gauge in the nose, and you push at the top to keep it round, you get a gasoline windshield wash to boot.

Not every airplane is made for this – I love to do parabolas and let a coin hover over the palm of my hand. In some cases, the engine sputters and stops when doing this.

The sum of inertial forces in the body frame of the aircraft is equal magnitude and opposite direction of the sum of all external (which for aircraft means aerodynamic forces and thrust) forces acting on the aircraft.

Per second law of motion, $a = \frac{F}{m}$. Since the aircraft is not accelerating in a reference frame attached to itself, the external and inertial forces must be balanced in it. They act as action and reaction pair regarding objects inside the aircraft like the slip-skid ball or the pilot.
Per general relativity, gravity is locally indistinguishable from acceleration of the reference frame and behaves as an inertial force for all purposes. Therefore external forces means just the aerodynamic forces and thrust (propeller thrust can be included in aerodynamic forces, but rocket thrust shouldn't be and for jet thrust it's somewhat unclear).

So saying you feel the aerodynamic forces or you feel gravity and centrifugal forces is equally valid and only a matter of point of view.

"Gravity is indistinguishble", it is certainly included in the "4 forces" to yield 0 acceleration steady state. "Thrust", no need to discriminate, it is a force acting on the aircraft for force vector analysis, particularly, as in many aircraft, it is angled down and to the right.  Let's not forget p forces for props, and, yes, gyroscopic forces (especially for single engine jets).

@RobertDiGiovanni, the principle of general relativity states: “effects of uniform gravity are indistinguishable from effects of acceleration of the frame of references”. That is the defining postulate of General Relativity, in its entirety. It means that the role of inertial reference frames from Newtonian mechanics goes to reference frames in free fall, and gravity should be treated as an inertial force (which are no longer called fictitious, because general relativity talks about all reference frames, and in those where they exist the inertial forces are real).

@RobertDiGiovanni, in the reference frame of Earth, the gravity is obviously included in the “4 forces” that yield 0 acceleration in steady state. But the reference frame of Earth is  in constant acceleration upward at $1g$ (it's actually gravity, but it is equivalent), and an accelerometer measures that $1g$ in it.

@RobertDiGiovanni,…  That reference frame is also rather uninteresting in this discussion, because it has some, but not all of the inertial forces. Either we use the body reference frame, where inertial forces—including gravity, because gravity is an inertial force—balance external forces, or we use the free-falling reference frame, where there are no inertial forces, but the aircraft is accelerating upward at $1g$ in it most of the time. Many things are simpler with the right choice of reference frame.

@Ad other forces, you are complicating what does not need to be. The point is just to define what are external forces on the aircraft. Mostly aerodynamic, but any reaction forces, and also gyroscopic forces, you are right, cannot be called aerodynamic, but are still external forces (even if the gyroscopic forces come from part of the aircraft, they are certainly external for the pilot and the accelerometer). In contrast to gravity, which is an inertial force.

Appreciate your responses, and yes, gravity treated as a force, period.  This brew-ha-ha (not a physics term), is about pilot perception, and in that regard, all unbalanced forces (acceleration, specifically lateral acceleration in a turn + gravity  = ball in curved tube location (airplane reference, may be banked)).  One writer seems to think gravity has nothing to do with this.  Attempting to assist.

@RobertDiGiovanni, there is no just “a force, period” here. There are external and inertial forces and you are making it more complicated by unnecessarily adding gravity on both sides of the equation. You say "- acceleration in turn + gravity = pilot perception"  where "acceleration" is due to "unbalacned forces" and that's correct (the minus is because gravity moves the ball in its direction, but acceleration moves it in opposite direction). But "unbalanced forces" are "external forces + gravity" (vector addition, so gravity cancels upward lift)…

@RobertDiGiovanni … combining the equations, you have "pilot perception = gravity - unbalanced forces = gravity - external forces - gravity = external forces" and gravity does indeed have nothing to do with this! Or everything, because properly the “local gravity” in the body frame includes the inertial forces due to acceleration. Because the terminology is somewhat confusing—“gravity” does not mean force due to gravitation, it really means all inertial forces. In the default Earth reference frame, it includes the centrifugal force.

I try to draw the vectors first before going right for the equations, and here it is important to realize that this is an issue of direction and magnitude.  In a loop, this would yield 2 Gs at the bottom and 0 G at the top.  In a horizontal turn, to "coordinate", we can match the rudder to the bank, bank less, or level entirely to "center the ball".  I use this (ancient) device when I am lifting off from the runway to control yaw force from the engine.  We can see when "wings level" (from visual cues), it measures lateral acceleration.  The tube will be more sensitive if it is less curved.

@RobertDiGiovanni, equations and diagrams are the same thing, just represented differently. It's about being comfortable switching between the reference frames and picking the one where it can be described most simply. Which in this case is the body frame (in the body frame, the aircraft is never banked, just the Earth can be anywhere, not only directly below).

@RobertDiGiovanni, the entire confusion is because everything here is driven by the choice of which frame of reference you are measuring things in. And anytime you measure things in an inertial (zero-G) frame of reference, gravity mysteriously disappears. The frame of reference fixed to the earth, (the atmosphere) is not an inertial frame of reference, it is an accelerated frame. If you tried to do the same measurements in a rocket ship undergoing one G of thrust in outer space, the results would be identical. The key is to realize that NO frame of reference is absolute. EVERYTHING is relative

We need to come up with additional fake "forces" like gravity, and centrifugal force, to explain things because we are attempting to measure acceleration in a  frame of reference (the earth) that is ALREADY ACCELERATING!

@Charles Bretana appreciate your philosophy.  Agree that centrifugal "force" is what you "feel" when accelerating in the other direction. However, please rethink what you said.  On earth, in the air, if you are not accelerating than what you are left with is gravity.  Very simple.  Some people confuse lack of velocity and zero acceleration.  I coined the term "aerogravimetric enviornment" to distinguish the effect of air drag balancing forces, leaving a 1 G condition.  The old ride in the elevator.

@CharlesBretana and others-- since comments are not for extended discussion on ASE, I've created a chat room for this topic -- https://chat.stackexchange.com/rooms/107177/lets-talk-about-gravity-can-you-feel-it

@RobertDiGiovanni, yes, IN THE EARTH FOR (Frame of Reference), "what you are left with is gravity". But you do NOT feel this. What you feel is the force of the lift on the wings. "Gravity" is the result of the frame translation tensor you would use to Translate answers derived on one FOR to another FOR when the two FORs in question are accelerating with respect to one another, To make things balance you need to apply an artificial "force" (which is unfelt) to make things balance. Take a look at https://en.wikipedia.org/wiki/Inertial_frame_of_reference

@RobertDiGiovanni, In particular, look at the second paragrapgh… "physics of a system vary depending on the acceleration of that frame with respect to an inertial frame, and the usual physical forces must be supplemented by fictitious forces". Honestly, I am not making this stuff up... Granted this is not necessary for the average GA pilot to understand, but you seem to me to be a bit above the caliber of the average GA pilot...

The only forces felt by a pilot are the aerodynamic forces from the pressure of the atmosphere on the surface of the aircraft, and the thrust of the engines.
Contrary to generally accepted concepts, Gravity is not a “force”. It is just an artificial construct we use to make the Math work out because we are generally measuring the motion of things (like an aircraft), in an accelerated frame of reference (FOR). In fact, if the aircraft was in free fall (Zero AOA and zero Lift) then the pilot would feel only the force of the engines and the force of aerodynamic form drag. When in “Level” flight, the pilot feels an additional “G” of force pushing on his butt because the airframe is generating additional Lift (an aerodynamic force perpendicular to the flight path vector and the earth) sufficient to maintain a constant altitude.
In a turn, the pilot does not feel “Centrifugal” force. “Centrifugal” force is another artificial construct or "fictitious force" we use because of measuring things in non-Inertial (accelerated) FORs. A rotating FOR is an accelerated FOR. What the pilot feels is the additional aerodynamic force on the airframe necessary to turn the aircraft (change it’s direction of motion) If you roll into a bank and do not pull harder on the stick/yoke, you will not generate any additional lift and you will not feel any additional force. Explaining this away by resorting to “Centrifugal” force, and then thinking about it as balancing some other fictitious “Centripetal” force just adds unnecessary complexity. All forces you feel are due to the aerodynamic forces pushing the surface of the airframe (and the engine).
In fact, if you read a bit about Gravity, you will learn that Newton's theory of Gravity, where it is represented as a "force", although it generates almost correct answers in 99.999% of the cases we are familiar with, is in fact absolutely wrong. Einstein showed, and proved, that gravity is the effect of mass distorting the curvature of space-time, in such a way as to cause everything traveling relative to an inertial (non-accelerated) frame of reference to "appear" to travel in a curved path.
In the FOR of the earth, "what you are left with is gravity". But you do NOT feel this. What you feel is the force of the lift on the wings. "Gravity" is the result of the frame translation tensor you would use to Translate answers derived on one FOR to another FOR when the two FORs in question are accelerating with respect to one another, To make things balance you need to apply an artificial or fictitious "force", (which is unfelt), to make things balance. Take a look at Inertial_frame_of_reference. In particular, look at the third paragraph, to wit:

In a non-inertial reference frame in classical physics and special
  relativity, the physics of a system vary depending on the acceleration
  of that frame with respect to an inertial frame, and the usual
physical forces must be supplemented by fictitious forces.
     * My Italics *

and the last sentence:

Another example of such a fictitious force associated with rotating
  reference frames is the centrifugal effect, or centrifugal force.

One of the coolest representations of this concept is in a book about Gravitation and shows how the paths of a thrown baseball, a bullet, and a beam of light, traveling from one point to another, will have exactly the same curvature when graphed in space-time, (in a one-G accelerated frame of reference),as opposed to their path in ordinary 3-D space.  If these were all graphed in a zero-G (inertial) frame of reference (Free-Fall) they would all appear to be traveling in an absolutely straight line.

Bottom line. All of the complexities and fictitious artificial "forces" we use to explain the forces acting on objects in motion are only necessary because we are measuring motion in some linear or rotationally accelerated frame of reference. We need to come up with additional fake "forces" like gravity, and centrifugal force, to explain things because we are attempting to measure acceleration in a  frame of reference (the earth) that is ALREADY ACCELERATING!  The only real forces acting on the aircraft are the force of the air pushing on the aircraft (aerodynamic forces) and the thrust of the engine[s].

Comments are not for extended discussion; this conversation has been moved to chat.

Throughout this answer we'll use "feel" or "felt" to mean the force or acceleration that is perceived by the pilot and transmitted through the aircraft structure as stresses or strains.  In a "weightless" situation, the net force or acceleration is not zero, but the "felt" force or acceleration is zero.
It is valid to argue that the force or acceleration "felt" by the pilot, and registered by the instruments, in flight is simply the net aerodynamic force or acceleration generated by the aircraft.  To get the actual net force acting on the aircraft, we have to add gravity to the aerodynamic forces.  In other words, the "felt" force or acceleration is the net force or acceleration acting on the aircraft, minus the force or acceleration due to gravity.
Examples--
1) Straight and level upright flight-- net acceleration zero, net force zero, "felt" acceleration 1-G in the upward direction, force "felt" by the pilot is simply equal to his weight, acting in the upward direction.  This force is the aerodynamic lift force generated by the wings, transferred through the aircraft structure to the bottom of the seat and then to the pilot's body.
2) Aircraft inverted at the top of a loop-- again the acceleration and force "felt" by the pilot are simply the result of the net aerodynamic force generated by the aircraft.  If the wings are generating positive lift, the pilot and G-meter will feel positive G's, and if the wings are generating negative lift, the pilot and G-meter will feel negative G's.  If the wings are generating zero lift, the pilot and G-meter will feel zero G's (weightlessness).
3) Steady-state coordinated turn-- the net aerodynamic force generated by the aircraft is simply the wing's lift vector, which contains no lateral (sideways) component in the aircraft's reference frame.  The slip-skid ball is centered.  Meanwhile the net force including gravity is the purely horizontal force vector-- the centripetal force vector-- that drives the turn.  This force vector does have a lateral (sideways) component in the aircraft's reference frame, yet the slip-skid ball is centered.
4) Turn with some sideslip-- the nose of the aircraft is allowed to point toward the outside of the turn.  The airflow impacts the side of the fuselage and generates an aerodynamic sideforce.  Now the net aerodynamic force generated by the aircraft does include a lateral (sideways) component, so the slip-skid ball rides toward the inside of the turn, and the pilot's body tends to lean toward the wall of the cockpit that is on the inside of the turn.
Yet if we take our airplane and bolt it to a roller coaster and run it through loops, flat turns, etc, the pilot still "feels" apparent forces, and the G-meter and the inclinometer still give indications.  What are causing these apparent forces?  (Assume for simplicity that the roller coaster is in a vacuum.)
We can say that these apparent forces are entirely due to the forces exerted by the rails of the roller coaster upon the wheels of the roller coaster, with gravity playing no role.  This is analogous to saying that the forces "felt" in flight are entirely due to the aerodynamic forces created by the aircraft, with gravity playing no direct role.
Yet we can also say that these apparent forces are entirely due to the combined effects of gravity, and of "centrifugal force".  Obviously, if we don't start a loop in the roller-coaster with enough speed to maintain positive G's all the way around, we'll hang upside-down in the seatbelts as we go over the top.
The key to reconciling these viewpoints is to understand exactly what "centrifugal force" is.  It is the apparent force created by a curvature in a vehicle's trajectory.  It is not a real force.  It is basically just the mirror image of the real forces that are acting to curve a vehicle's trajectory.  For (almost) every centripetal force acting to drive a curvature in a vehicle's trajectory, we can say the occupants "feel" an apparent "centrifugal force" acting in the other direction.  When we do a high-speed turn to the left in a car on flat ground, the tires generate a real force toward the left, and we say we perceive a centrifugal force to the right, and are thrown against the right wall of the inside of the car.  If we want to understand which direction a slip-skid ball would deflect, we could consider the real centripetal force from the tires, or the apparent "centrifugal force" generated by the resulting turn rate, but not both.  If we consider both forces, we end up with zero net force-- they would cancel each other out-- predicting that the slip-skid ball would not deflect at all.  This would be wrong.  We can't mix and match apparent "centrifugal force" with real centripetal force.  We have to use one or the other.
In this context, we're not using "apparent" to mean "the felt component of".  We're just using "apparent" to remind ourselves that "centrifugal force" doesn't actually exist.
We stated that "For (almost) every centripetal force acting to drive a curvature, we can say the occupants 'feel' a "centrifugal force" acting in the other direction".  Why "almost"?  Because we can't "feel" any centrifugal force that is caused by a curvature in a vehicle's trajectory that is due to gravity.  (Consider the forces felt by an astronaut in orbit.)
Why is gravity such a special force?  Forces from wings and forces from wheels must be transmitted through the aircraft structure and to the exterior of the pilot's body.  As a result, stresses and strains are created in the aircraft structure and in the pilot's body.  In contrast, gravity exerts an equal acceleration per unit mass on every molecule of the aircraft structure, and on every molecule of the pilot's body, creating no stresses or strains, i.e. no displacement of one part of the structure or body relative to another part.  Likewise, unless we consider the outer housing of the slip-skid ball or the G-meter to be rigidly fixed in space, gravity has no tendency to displace any element of either of those instruments in relation to any other element.  For example, gravity has no tendency to pull the inclinometer ball to one side within the glass tube, even if the tube is tilted relative to the earth.  (We're assuming that our instrument is much smaller than planetary in scale, so that tidal effects may safely be ignored!)
We've defined "centrifugal force" as an apparent force that is the mirror image of the net centripetal force acting to drive a curve in a vehicle's trajectory.  If the force of gravity is not "felt", then what is the "felt" component of that apparent "centrifugal" force?
Since gravity is already "built in" to the net centripetal force determining a vehicle's trajectory, it follows that an apparent upward force opposite to gravity is already "built in" to the apparent "centrifugal" force that is paired with that centripetal force.  This means that the "felt" centrifugal force is equal to the total centrifugal force plus gravity.  Just as the "felt" centripetal force is equal to the total centripetal force minus gravity.
(Note-- it's really only the component of gravity that acts perpendicular to the flight path that is "built in" to the centripetal force vector.  For simplicity, in the discussion of "centripetal force" in this answer, we'll assume that we're looking at the vehicle's trajectory at a point where the trajectory is exactly horizontal rather than rising or falling, and thus the full weight vector acts perpendicular to the flight path.  Alternatively, we could think in terms of a three-dimensional vector sum rather than a two-dimensional one.)
The key point here is not to argue about whether "centrifugal force" actually exists or not.  Rather the key here is to recognize that we're starting from a situation where the exact trajectory is known, and then we're calculating the centripetal (or centrifugal) force that is involved in creating this trajectory, and then we're subtracting away (or adding) gravity to get the "felt" component of that centripetal (or centrifugal) force.
In essence, "centripetal force" is just another way to say "net force", except that we're discarding components that act tangential rather than perpendicular to the flight path.  In the context of flight, the aerodynamic forces and the force of gravity are both built in to the concept of "centripetal force".  "Centrifugal force" is an apparent force or pseudoforce that is equal and opposite to the real "centripetal force".
In the case of a roller coaster, the "felt" component of the centripetal force would be the force exerted by the tracks on the wheels, and the "felt" component of the centrifugal force would be equal and opposite.  In the case of an aircraft, the "felt" component of the centripetal force would be the aerodynamic force created by the aircraft, and the "felt" component of the centrifugal force would be equal and opposite.  Whether we want to say that the pilot, G-meter, and inclinometer are responding to the "felt" component of the centripetal force or the "felt" component of the centrifugal force is really just a matter of convention, but only the former actually exists.
So all these three things are true:
1) "Felt" force = aerodynamic force = net force minus gravity
And
2) "Felt" force = net centripetal force minus gravity
And
3) "Felt" loading (the sensation of apparent weight due to being accelerated by a force) = ( net "centrifugal" force plus gravity ) / mass
We're taking the convention here that the felt "loading" acts in the opposite direction to the felt "force" or felt "acceleration", just as the apparent "centrifugal" force acts in the opposite direction to the actual centripetal force.
Whether we want to work in terms of "centripetal" or "centrifugal" forces is really just a matter of convention.  Only the "centripetal" forces are actually real.  To get the "felt" component, we subtract gravity in the former case, and we add gravity in the latter case.  Because of the way that gravity can't actually be "felt", yet it is already "built in" to the centripetal and "centrifugal" forces.
Note also that by specifying "centripetal" (or "centrifugal"), we're saying that we're limiting the force and acceleration components of interest to those acting perpendicular (orthogonal) to the flight path, and disregarding those acting parallel (tangent) to the flight path.  The first equation in the list of three above does not have that constraint.
Often one sees diagrams in flight training manuals that try treat "centrifugal force" as just one component out of several forces acting to curve the flight path.  Many diagrams have been published, including by the FAA, that illustrate the deflection of the slip-skid ball in turning flight as being due to a force balance between "centrifugal force", and "gravity", and lift, or between "centrifugal force" and lift.  (Oddly, the real aerodynamic sideforce caused by the fuselage flying sideways through the air is invariably omitted.)  At first this seems intuitively right-- if you are bringing gravity into the picture rather than just purely considering centrifugal force alone, then why not bring lift into the picture too?  But this approach just doesn't work. The effect of the lift vector is already included in the centrifugal force vector.  To explain the forces "felt" in a coordinated or uncoordinated turn, we need to work in terms of the real aerodynamic forces, or we need to work in terms of the balance between "centrifugal force" and gravity, which is essentially the same as working in terms of the balance between centripetal force and gravity.  A diagram combining gravity, centrifugal force, and lift just can't hold water.
If the flight path is fully constrained, then the "centrifugal force plus gravity" (or "centripetal force minus gravity") approach works well.  For example, if the flight path is known to be linear, then centrifugal force is known to be zero, and the slip-skid ball works as a bubble level, telling us which way is down.
But in many situations, the flight path is not constrained to be linear.  Also, unlike the case of the roller coaster, the turn radius is free to vary as the lift force is varied, and the flight path is free to curve up and down in the vertical plane as well.  That is why it is so useful to take the viewpoint the forces "felt" in flight are simply the real aerodynamic forces generated by the aircraft, with gravity and "centrifugal force" playing no direct role.
Consider for example this related ASE answer about why a glider experienced a negative G-load at the top of the loop.  If we imagine the loop to be constrained to follow a pre-defined track with as set radius like a roller coaster, we could imagine that the explanation is simply that we didn't start with enough speed to generate enough "centrifugal force" to counteract gravity and "make it all the way around" without hanging loose in the belts.  But recognizing that the flight path is not constrained to follow a pre-defined track, it is more useful to recognize that when we "felt" a negative G-loading, the wing must have been flying at a negative angle-of-attack and generating negative lift.  Which raises the question, "Why was the wing generating negative lift-- with the stick full aft?  Why didn't the angle-of-attack stay positive, as the pilot was intending to command, even if the resulting trajectory did not result in a perfectly round loop?"  The negative-lift situation is in fact related to the low airspeed at the top of the loop, but for reasons that are much more subtle than the fixed-track roller-coaster analogy would suggest.
The idea that a coordinated turn features a "balance" between centrifugal force and gravity may be technically true, but it has no explanatory power, especially in the context of a flight path that is free to vary in three dimensions as lift and other variables are varied, rather than being constrained to run along a pre-existing track as in the case of a roller coaster.  It doesn't reveal that the key factor in a "coordinated" turn is simply that the fuselage is streamlined to the airflow, so that the aircraft is generating no aerodynamic sideforce and thus has no aerodynamic force component in the lateral (sideways) direction in the aircraft's reference frame.  It can lead one to believe that in a sideslip, an abnormally low turn rate is somehow causing the aircraft to slide sideways through the air, rather than understanding that the aircraft is being forced or allowed to fly sideways through the air which is creating aerodynamic sideforce (as well as drag), which is in causing a reduction in the turn rate.  It can lead one to believe that a pilot should somehow be making adjustments in angle-of-attack to get the "right" lift force for the airspeed and bank angle, or else the aircraft will go sliding off toward the inside of the turn.  (Footnote to be added.)
In short, the "centrifugal force plus gravity" approach is often not the most useful way to look at the problem when the flight path is free to vary rather than constrained to be some particular shape like a straight line, or a perfectly circular loop of pre-defined radius, or a round horizontal circle with a pre-defined turn radius and bank angle or airspeed.  Unless we are back-engineering the "felt" forces (i.e. the aerodynamic forces) from a saved track log that was recorded with tremendous precision, the "centrifugal force plus gravity" approach doesn't have much application.  To know what the "centrifugal" force is, we have to know what the trajectory is, and to know what the trajectory is, we usually have to know the forces acting on the aircraft, including the aerodynamic forces, in which case we already know the answer to question of what will be the result of "centrifugal force plus gravity".
The "centrifugal force plus gravity" approach is generally better suited to understanding the forces felt in race cars travelling along tracks of fixed bank angles and fixed turn radii, or the angle taken by a rope of fixed length tied to bucket full of water swung round in a circle, than to the forces "felt" in an airplane whose path is free to vary in three dimensions.  But when applied properly, it is not an invalid way to look at the mechanics of flight.

Sorry, tried to answer in chat, blanked out again.  Easy to fly a constant G circle in the horizontal.  In the vertical loop,  you would have to vary speed and in a perfect circle, which would involve variable thrust.  Notice the difference, in the horizontal circle, the gravity vector does not change, in the vertical, one hangs from the harness at top without pulling.

See also chat room for this topic -- https://chat.stackexchange.com/rooms/107177/lets-talk-about-gravity-can-you-feel-it

No such thing as "actual" force!!!!- all forces are the result of a calculation done in some specific frame of reference. And the answer you will get from the calculation depends completely on which frame if reference you do the calculation in. For measurements of forces, the accelleration that the frame of reference is experiencing will change the answer. Its exactly analogous to calculating your position.... Until you specify the frame of reference, no answer is meaningful. Asking where are we? And getting the answer 90 miles, tells you nothing. You have to know where position 0 is.

For measurements of forces, the acceleration that the frame is experiencing will shift the answer you get from any force calculation. And because the frame we.commonly use (fixed to the earth), is not a zero-G inertial frame, it is accelerating upward at 32ft/sec2, any measurements are shifted by this amount. It's exactly the same as if you measured it in a rocket in outer space accelerating at one G.

@CharlesBretana , you have a lot of good insights and I don't necessarily disagree with anything you say.  I might be just tackling the problem from a little lower level, more aligned to "common sense" or the forces that we seem to perceive in everyday life, than you are.

That's a fair comment. It is true that the "real" answer to this issue requires an understanding of the world that is very different from the "common sense" generally understood perception most of us have. To top it off the "common sense" explanation works! Although incorrect, and, unfortunately, much more complex than the correct approach, it gives the right answer and does not create the immediate visceral negative reaction that the "correct" explanation seems to generate.

Quiet flyer, thinking more about your answer. Everything you present is logical and adds up consistently. Trying to reconcile it with what I understand, I realized why. What you are doing is explaining everything as it would be seen (measured) in the one "G" rotating frame of reference attached to the aircraft. In that frame a g-meter would show one G of acceleration up and whatever the acceleration of the turn. THATS why you need to include gravity and centrifugal force in your analysis. They are, simply fictitious forces added to compensate for the acceleration of the frame of reference.


text:
Does it ever happen that increasing the prop RPM without moving the throttle actually decreases the thrust and power delivered to the airframe?
A) In an airplane with a piston engine and a constant-speed prop, flying at some given airspeed with the manifold pressure and RPMs in a typical cruise configuration, would it ever happen that increasing the prop RPM higher and higher without moving the throttle would at some point actually start to decrease the thrust and power delivered to the airframe?
Assume no electronic engine controls such as FADEC.
I'm imagining a case where flattening the pitch of the prop blades would decrease the thrust and power delivered to the airframe, because the prop blades would be meeting the local airflow at a lower angle-of-attack, despite the increase in RPM.  I guess an extreme case would put the prop blade at a small negative angle-of-attack to the local airflow, so that the prop was acting to drive an increase rather than a decrease in the engine RPM-- obviously this would increase the RPM but would decrease the thrust and power delivered to the aircraft, at least in the forward direction.
B) It occurs to me now that such a thing might happen at lower throttle positions but not at higher throttle positions.  So a second part of this question is, on final approach with the throttle at idle, is it true that the prop exerts more of braking effect on the airframe when the prop lever is set for high rpm than when the prop is set for low rpm?
C) And the third part of this question is, for some given airplane cruising at some given airspeed with RPM and manifold pressure initially "square", what is an example of the highest initial manifold pressure at which we might find that advancing the RPMs higher and higher without changing the throttle setting might eventually cause a drop in the power delivered by the prop to the airframe?  Or is this something that would only happen at very low power settings, such as would be used in a descent, and would never happen at any power setting capable of sustaining level flight at any airspeed?  (This is basically a more refined version of part A.)
The intent of the question is that the advance in the RPMs is intentional and goes no further than the max allowable RPM of the engine and prop.  Clearly a "runway" prop would be an extreme example of these dynamics but the question is intended to be constrained to the normal allowable operating envelope of the prop and engine.
Some related questions and answers on ASE--
Manifold pressure vs rpm vs power obtained and more about constant speed prop
On a constant speed propeller, how is blade pitch related to airspeed and efficiency?

You may get a speed increase but it's likely to be almost too small to detect (although this depends a lot on the plane and propeller combination).
When you increase prop RPM leaving MP constant, you are increasing the engine brake horsepower although there is a small drop in MP when you do that (and the opposite happens when you reduce RPM) that limits the total horsepower increase.
Say you have a Lyc O-360. Running it at "23 square", 2300 RPM and 23" MP, the engine is making roughly 108 HP at the prop.  If you raise the prop to 2700 rpm from 2300, and the MP stayed at 23", you are now making about 128 HP.  But actually it's not quite, because increased velocity demand of the intake flow drops the MP a bit with the throttle butterfly unchanged.  If you went from 2300 to 2700 the MP would drop somewhere around 1" of MP and this would reduce the engine's brake HP to 120.  Still more than 108.

At the high RPM, you will also likely have the prop operating at a less efficient blade angle from low blade AOA - the governor doesn't know or care what the blade angle is, it just rotates them to load to blades to keep the RPM where it's set by the prop control (a spring pushing on a set of flyweights) so there will be another minor loss from the blades being at a sub-optimal AOA.  Say, optimistically, 3%. So you end up with the equivalent of maybe 112-115 HP in net thrust compared to what you had before (this can vary a lot with different propellers).
So the result is, in a best case condition, you actually only increased the net thrust by a few percent and taking into account of the power-required-increases-at-the-square-of-the-speed aspect, there is a net airspeed increase of maybe a couple mph at best. If the prop efficiency really dropped off badly, you might find yourself going slower by a knot or two.
It also matters how you do it. If you were cruising along and raised the RPM suddenly by shoving the control forward, you will get a temporary deceleration as the governor unloads the blades to raise the RPM (it will also overshoot the redline giving that WEEEEEEEEOOWWWWWWWW sound you sometimes hear when pilots do that) and then the plane will slowly catch up.
If you just raise the RPM gradually, the proper way to do it, there should be little obvious acceleration or deceleration, but the speed may creep up or down a little bit and after things settle down you'll be a few knots faster, the same, or maybe a couple knots slower because the prop was operating at a very inefficient blade angle and the losses exceeded the minor Brake HP increase of the rise in RPM.
If you're at a very low manifold pressure setting, just off idle say, and the prop control is at Max RPM, the prop governor has a "command" (fly weight spring pressure) to let the engine run to redline and regulate RPM there, but the engine isn't making enough torque to spin the blades at redline.  The governor will unload the blades trying to do that, until the blades reach the fine pitch limit.
Or, if you're at full throttle/Max RPM and pull the throttle way back, at some point the blades hit the fine pitch limits and the RPM starts to drop from redline.  The prop has "come off the governor".  Now the prop is effectively an overly fine fixed pitch prop.  If you are at a certain fairly low MP, say 16", with the RPM at, say, 2200, and you move the prop lever up to Max RPM, the RPM may rise just a little as the blades were already near the fine pitch stops to keep 2200 RPM at 16". Add throttle, and the RPM rises like a fixed pitch until the RPM matches whatever RPM the prop lever is set to, and it "comes back on the governor" as the blades are moved off the fine pitch stops to keep the RPM at the selected setting.
As a general operating technique, you always lead with RPM on power increases, and lead with throttle on power decreases.  And you get to know that each RPM change affects MP and learn to allow for it with each RPM change. For example, if you are climbing at 24 square and want to cruise at 22 square, you'll reduce the MP to about 21.5-ish, then reduce RPM to 2200, knowing that this will pick the MP up to 22"-ish, requiring a minimal subsequent throttle tweak to get it right on.

This may be a condition known as "runaway prop", the opposite of "feathering".  As the pitch becomes finer, RPM goes up, but aircraft drag greatly increases, and thrust progressively decreases as pitch gets farther and farther away from optimum AOA.
This is a malfunction of the pitch control system.
Intentionally reducing pitch (for whatever reason) with the same power input would cause an increase in rpm.  There is the separation of power (essentially rate of fuel burn) from prop load (essentially prop drag created at a given AOA and RPM, which determines thrust).
$Power$ is defined as $Force$ × distance/time.  $Thrust$ × distance/time is defined by $Power$ × $Efficiency$.   The decrease in $Thrust$ is a result of a loss in prop efficiency.  A pilot may try firewalling the throttle to hold speed, resulting in more $Power$ from the pistons, but to no avail.  The increased drag and decreased thrust would result in a loss of airspeed.  The increased energy output would register at a rise in engine temperature.

Good point about the runaway, I should have thought of that when I was writing the question.  Hope you don't mind the latest clarification, I can roll it back if you feel it invalidates your answer but I don't think it really does.

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text:
Is some sort of clearance, such as a clearance to enter nearby controlled airspace, required to legally fly in IMC in Class G airspace?
In the US, is some sort of clearance, such as a clearance to enter nearby controlled airspace, required to legally fly in IMC in Class G airspace?
Or to put the question another way, in the US, may an instrument-rated pilot flying an IFR-equipped aircraft in Class G airspace violate the VFR cloud clearance requirements without an IFR clearance, without violating any FARs other than (the rather ambiguous and subjective) FAR 91.13?
Does it make any difference to the answer if the pilot is NOT following any specific established instrument approach or departure procedure?  For example, if the pilot is taking off from an airport for which no instrument approaches and departures have been established?  Or if the pilot is flying in one of those now-rare bits of airspace where the class G ceiling is well above 1200' AGL?

This topic has been been addressed at length before on ASE but never has actually been asked as a specific question; I figured it deserved to be. For related discussion see https://aviation.stackexchange.com/questions/21888/can-an-ifr-clearance-be-issued-and-flown-through-imc-in-class-g-airspace and https://aviation.stackexchange.com/questions/78751/could-a-flight-ever-be-considered-to-be-operating-under-visual-flight-rules-and and https://aviation.stackexchange.com/questions/78746/does-a-glider-pilot-need-to-file-an-ifr-flight-plan-to-legally-fly-in-cloud-in-c .

Also see https://aviation.stackexchange.com/questions/47569/can-i-go-into-clouds-if-i-got-ifr-clearance-uncontrolled-airport (all answers), and https://aviation.stackexchange.com/a/45324/34686

An FAA-appointed Designated Pilot Examiner has recently told me that it is legal to fly an airplane or glider in clouds in Class G airspace with no clearance, if the aircraft is appropriately equipped and the pilot has an instrument rating in airplanes.
On the other hand, here is a case, originally noted in an answer to a related question on ASE, where a pilot  who took off from
an uncontrolled airport into clouds without a clearance or
release from air traffic control (ATC) was found to not be in violation of FAR 91.155(a), the VFR cloud clearance and visibility requirements, but was found to be in violation of FAR 91.13(a), the prohibition against operating an aircraft "in a careless or reckless manner so as to endanger the life or property of another."  An interesting aspect of the pilot's defense in this case is his suggestion that by departing immediately after another aircraft which had received an ATC release and clearance, he assured himself that there would be no other IFR aircraft in the airspace above the airport 1.  Other aspects of the pilot's defense were the fact that  he broadcast his departure intentions on the Unicom frequency, and he monitored the ATC frequency for other traffic.
The FAA and NTSB did not find these arguments to be convincing.
There definitely seems to be a grey area here as to exactly what precautions, other than receiving an IFR clearance for adjacent controlled airspace, would satisfy the FAA as far as FAR 91.13 is concerned, in the context of operations in IMC in uncontrolled airspace.  Also, one can easily imagine situations that are clearly compliant with all regulations and all accepted practices, and yet clearly create significant risks, arguably more so than the case linked to in this answer.  For example, one could argue that any time any aircraft operates in IMC in uncontrolled airspace, this creates a significant risk of collision with VFR traffic which may be legally flying just outside the edges of clouds with no radio or transponder.  Yet IMC in uncontrolled airspace is an accepted practice-- many airports are located in airspace that is Class G up to at least 700' AGL, yet have established instrument approaches with minimum decision heights much lower than that.
Footnotes--
1-- Page 4 of the finding says that the respondent claimed that by following this procedure the respondent "assured himself that there would
be no other IFR aircraft in the controlled airspace above the
airport".  This may be a typo-- it should perhaps read "uncontrolled" rather than "controlled"-- since there seems to be no dispute that the respondent was able to observe the prescribed VFR minimums once he reached controlled airspace.

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text:
Why did the Wright brothers take steps to increase the amount of vertical surface area forward of the CG on some of their aircraft?
Why did the Wright brothers take steps to increase the amount of vertical surface area in front of the CG on some of their aircraft?
Could this have actually improved the flight characteristics in any way that could not have been equally well accomplished by slightly reducing the size of the aft-mounted vertical fin or all-moving rudder?
(End of actual question)

Supplemental content that may help inform an answer--
If the goal was a decrease in "weathervane" stability1, wouldn't reducing the size of the vertical fin or rudder aft of the CG have given the same result, with less drag?
If the goal was to prevent sideslip, wouldn't increasing the vertical surface area forward of the CG actually be counterproductive?  Wouldn't increasing the vertical surface aft of the CG have given the desired result?
Did the Wrights have a notion of that sideslips were caused in part by gravity trying to "pull" a banked aircraft sideways through the air toward the low wingtip, while the aircraft had insufficient side area to resist this "pull"?  If so, wouldn't this have been a faulty notion of the fundamental cause of sideslips, and of the balance of forces in a non-slipping (coordinated) turn?
Or did the Wright brothers have some conception that it was desirable to maximize the sideforce generated during a sideslip for some other reason?  If so, does this conception really make any sense from a modern perspective?
Did the Wright brothers have a misconception of the basic nature of spiral instability, leading them to mistakenly suspect that an increase in the sideforce generated by sideslipping, rather than a decrease in "weathervane" stability along with an increase in dihedral, would be the most productive approach toward making an aircraft less prone to "winding up" into a steeper turn?
The Wrights left many letters and other written notes, so it should be possible to answer these questions with some degree of authority.
Related quotes from outside sources:
"1905 Flyer III:  Semi-circular [vertical] "blinkers" between the surfaces of the canard prevent the nose from dropping in a turn.   With this aircraft, the Wrights were able to fly until their fuel ran dry. In 1908, they adapted the Flyer to carry the first airplane passenger." -- From http://www.wright-brothers.org/Information_Desk/Just_the_Facts/Airplanes/Wright_Airplanes.htm
"The crash convinced the Wrights to make radical changes to the aircraft design. They almost doubled the size of elevator (in front) and rudder (in back) and moved them about twice the distance from the wings. They added two fixed vertical vanes (called "blinkers") between the elevators to serve as stabilizers and help prevent the Flyer's tendency to slip or slide sideways in a turn." -- from http://www.wright-brothers.org/Information_Desk/Just_the_Facts/Airplanes/Flyer_III.htm
Photographic examples:
1)http://wrightbros.org/History_Wing/Wright_Story/Airplane_Business/Airplane_Business_Intro_images/1911_Exp_Glider.jpg , from http://wrightbros.org/History_Wing/Wright_Story/Airplane_Business/Airplane_Business_Intro.htm
2)http://www.wright-brothers.org/Information_Desk/Just_the_Facts/Airplanes/Wright_Airplane_images/1905_Flyer_III/1908_Flyer_III_at_Kitty_Hawk.jpg , from http://www.wright-brothers.org/Information_Desk/Just_the_Facts/Airplanes/Flyer_III.htm
3)http://www.wright-brothers.org/Information_Desk/Just_the_Facts/Airplanes/Wright_Airplane_images/1905_Flyer_III/1905_Flyer_before_rebuild.jpg , from http://www.wright-brothers.org/Information_Desk/Just_the_Facts/Airplanes/Flyer_III.htm
4)http://www.wright-brothers.org/Information_Desk/Just_the_Facts/Airplanes/Wright_Airplane_images/1907_Model_A/1909%20Launch%20in%20Italy.jpg , from http://www.wright-brothers.org/Information_Desk/Just_the_Facts/Airplanes/Model_A.htm
5)http://wrightbros.org/History_Wing/Wright_Story/Airplane_Business/Airplane_Business_Intro_images/Vin_Fiz_Takes_Off.jpg , from http://wrightbros.org/History_Wing/Wright_Story/Airplane_Business/Airplane_Business_Intro.htm
6)http://www.wright-brothers.org/Information_Desk/Just_the_Facts/Airplanes/Wright_Airplane_images/Model%20CH/1913_CH_at%20anchor.jpg , from http://www.wright-brothers.org/Information_Desk/Just_the_Facts/Airplanes/Wright_Airplanes.htm
Footnotes --
1-- In this question, "weathervane stability" is used to mean "yaw stability", i.e. "directional stability". No implication is intended that an aircraft in flight "feels" the external, meteorological wind, apart from the effect of sudden gusts.
Addendum-- it seems possible that in some cases, part of the motivation for installing these vertical surfaces may have had to do with the Wright brothers' experiments with an automatic stabilization system, which included a pendulum, which would only give useful indications if the aircraft tended to generate a significant amount of sideforce whenever it was banked and turning.  So the present question is confined to aircraft that were not being used for experiments with the automatic stabilization system and were not given specific design features for the purpose of helping the automatic stabilization system work better.

There are a lot of questions here.

@dalearn -- thanks, edited to try to help address that

There was much controversy at the time over whether a flying machine should be statically stable in flight. It was known that a high degree of stability meant that the machine would sit in one attitude and be hard to manoeuver. The Wrights were firmly in the camp who believed that for a plane to be manoeuvrable, which is to say controllable, it would need to be unstable. This was why they put the elevator in front. According to their theory, a large control rudder at the rear would need a destabilizing fin at the front to counter its stabilizing tendency.
When Orville was over in England in 1910, as an honorary member of the Aeronautical Society of Great Britain he and his UK lawyer Griffith Brewer acted as official witnesses to the certification flight of the world's first stable airplane, the Dunne D.5. During the flight Dunne took his hands off the controls and wrote on a piece of paper provided by Brewer, before flying a full circuit and landing back near his takeoff point. Flight published an account and a facsimilie of Dunne's note. Orville was stunned both by Dunne's use of elevons (for it was a tailless swept biplane!) and by the ability of such small control surfaces to have such a great ability to control the machine in flight. It was one of the events which led to the Wrights abandoning forward auxiliary surfaces altogether at around that time.

Weren't already Liliethal gliders stable and the Etrich-Wels glider of 1906 the first stable tailless aircraft?

The usual context when discussing the Wrights and their work is that of powered, controlled flight. In this context, weight-shifting is excluded as a permissible control mechanism. From your wider perspective, the first stable aircraft was a Montgolfier balloon. The first stable (but uncontrolled) manned heavier-than-air glider was built by George Cayley and "manned" by a boy. He later pressured his coachman into a controlled glide. Lilienthal, Biot, Pilcher and Weiss were relative latecomers. The manned kites of Hargrave, Baden-Powell and Cody were also aircraft and were stable.

At your level of diligence, the first stable aircraft was Indian. Wouldn't it be better to stick with verified facts?

@PeterKämpf. From your link: "The Sanskrit text that describes the ancient flying machines which Bodas cites, the Vaimanika Shastra, has itself been studied thoroughly by scientists who have concluded that very few of the craft it describes would likely be able to fly at all." I have to say, you normally do better than this.

Biographies containing accounts of the Wright brothers' conversations at Kitty Hawk while developing the control system of their early gliders seem to suggest that they had some confusion about the difference between a sideslip and a spiral dive.  They first encountered the problem of "adverse yaw" with their 1902 glider, and added a fixed vertical fin at the rear of the aircraft to counteract this, but reported that this seemed to make the aircraft even less controllable.  A modern description of the problem might be that the fin converted the aircraft from one prone to experiencing extreme adverse yaw and sideslip, to one prone to experiencing spiral instability due to excess directional stability and resulting lack of sideslip.  It's not clear that the Wrights ever thought about the problem in these terms.  The situation was complicated by their choice to use an anhedral geometry on their early gliders and powered aircraft, to avoid a tendency for the aircraft to roll downwind when struck by a crosswind gust.  By the time the Wrights were adding vertical surface area to some of their aircaft in front of the C.G., they had abandoned the anhedral geometry.
It seems unlikely that adding vertical surface area in front of the C.G. really changed the stability or control characteristics of the Wright aircraft in any beneficial way that could not have been accomplished with less drag simply by reducing the size of the rear-mounted vertical fin or all-moving rudder.  In fact, it seems rather unlikely that adding vertical surface area in front of the C.G. really had any beneficial effect on the aircrafts' stability or control characteristics at all.
As a general rule, a modern designer would never take steps to intentionally increase the amount of vertical surface area in front of an aircraft's C.G., notwithstanding a few exotic exceptions where a moving rudder control surface was placed in front of the C.G..

Still would reconsider the, shall we say, forward half delta.  With head on wind it is smaller in area than the vertical stabilizer/rudder assembly, itself a squareish flat plate.  This would be directionally stable.  In a crosswind, the "assembly" weathervaning force would stall, whereas the little half delta in front would keep "lifting", helping reduce the weathervane.  If course the other way is opposing rudder input, but at the very slow speeds the Wrights flew, one can see why they chose the "jib".

I can see a beneficial aspect: What that forward area adds is yaw damping. Maybe this created the desired effect, making the Flyer more sluggish in its yaw movements. By distributing areas on both sides of the center of gravity, the Wrights could independently tailor directional stability and damping, something we today do by choosing the right fuselage length.

@Peter Kampf Copy that, see the swept wings and forward fuselage of the 727.  Fuselage side area and shape make for some interesting possibilities.  May be why Bleriot did not cover the aft fuselage, though these days we know covering can make the structure much stronger.


text:
Could a flight ever be considered to be operating under Visual Flight Rules and Instrument Flight Rules at the same time?
In the US, could a flight ever be considered to be operating under Visual Flight Rules and Instrument Flight Rules at the same time?
If not, where in the FARs is this spelled out?
Consider the specific case of an IFR-rated pilot flying an IFR-equipped aircraft, but with no IFR or VFR flight plan filed and no IFR clearance requested or delivered, staying in Class G airspace and observing the VFR cloud clearance and visibility requirements spelled out in FAR 91.155.  How can we determine whether this flight is operating under VFR or IFR?  Why could it not be considered to be operating under both?
Bonus question-- is there any specific case, falling within the general conditions described above, where it would matter whether the flight were operating under VFR or IFR?  Assuming that the pilot intends to, and is able to, remain in Class G airspace and observe the VFR cloud clearance and visibility requirements spelled out in FAR 91.155, would it be practical for the pilot to decide in advance that he will simply flip a coin to determine the answer if ATC, or anyone else, enquires whether he is operating under VFR or IFR?  With the added proviso that if no one makes such an enquiry, he will leave the coin unflipped until after landing, at which point he will flip the coin to see whether the flight was conducted under VFR or IFR?  Would such a plan present any practical difficulties?
Related, but not the same--
Can an IFR clearance be issued and flown through IMC in class G airspace? -- note that several of the answers, as well as many comments, to this question also address a different question (which probably ought to be asked as a separate ASE question), which is "May an IFR-rated pilot flying an IFR-rated aircraft in Class G airspace violate the VFR cloud clearance requirements without an IFR clearance?"

Sure, if the pilot's name is Schrodinger...

P.S. I downvoted specifically for the ridiculous bonus question about flipping a coin after the flight to determine whether it was VFR or IFR.

@MichaelHall -- the "ridiculous bonus question" was meant to make people stop and think twice about the assumed exclusivity of VFR vs IFR

@MichaelHall -- ... and also to mirror the ridiculousness of the FAA sanctioning a pilot for flying under IFR in IMC in Class G airspace, while routinely condoning IFR flight in IMC in Class G airspace under circumstances that are really only slightly different and still offer no protection against collision with some poor guy legally flying an aircraft with no electrical sytem, radio, or transponder just outside the edge of a cloud.  For more see https://aviation.stackexchange.com/questions/21888/can-an-ifr-clearance-be-issued-and-flown-through-imc-in-class-g-airspace/78080#78080

@MichaelHall -- the ridiculous bonus question was meant to make people stop and think twice about the assumed exclusivity of VFR vs IFR

@MichaelHall -- ... and also to mirror the ridiculousness of the FAA sanctioning a pilot for flying under IFR in IMC in Class G airspace, while routinely condoning IFR flight in IMC in Class G airspace under circumstances that are really only slightly different and still offer no protection against collision with some poor guy legally flying an aircraft with no electrical sytem, radio, or transponder just outside the edge of a cloud.  For more see https://aviation.stackexchange.com/questions/21888/can-an-ifr-clearance-be-issued-and-flown-through-imc-in-class-g-airspace/78080#78080

No, you can't be both VFR and IFR at the same time. It would be like saying that a ball is both red and blue at the same time. You are either one or the other, never both.
If the pilot hasn't received an IFR clearance from flight services, then they are operating under visual flight rules, regardless of their intentions, capabilities, or flight plan. The sole exception is in class G airspace, specifically when they're flying a pre-set departure or arrival procedure from an airport. If a VFR pilot enters a cloud (and there's no emergency in progress), then they are breaking the law, and they can't just decide later that "oh, I was actually IFR at that time".
ATC is never going to inquire about which rule set you're operating under unless there's some kind of administrative foul-up. They're the ones that either did or did not issue you the clearance, so they already know which rule set you're operating under.

One can operate under IFR in class G without a clearance or even a flight plan. So, unless/until you enter a cloud, it can't be established which rules you are using. And, if you don't do so, it is theoretically possible to comply with both at the same time.

@StephenS, are you an instrument rated pilot or air traffic controller?

@StephenS Yes, I did mention that exception in my answer. And, yes, it's possible to comply with both sets of rules, but that doesn't mean you're actually operating under both sets of rules. Some highways in this country have a speed limit of 60mph, and some have a limit of 70mph. If I drive 60, I'm technically complying with both sets of rules, but that doesn't mean that the speed limit is somehow both 60 and 70 at the same time.

@StephenS, if you were to operate IFR in class G without a clearance, what squawk code would you use?

@MichaelHall If you're taking off IFR from an airport in class G airspace, you wouldn't get a squawk code until you get in contact with ATC. So it doesn't matter what you put in your transponder.

@HiddenWindshield, when I have done this I have called on the phone and gotten a clearance and squawk before departing.  Anyway, that isn’t my question.  My question is for those who claim that you can take off under IMC and conduct the entire flight in class G airspace in IMC without a flight plan, and without talking to ATC, and the flight would be considered IFR.

@MichaelHall I see what you're saying. That's certainly the safest way to do it. But it is possible to take off IFR into class G airspace, and not get a clearance or squawk code until you contact ATC during climbout before you enter class E/C/B/whatever airspace.

And as long as you adhere to VFR cloud clearance requirements there is no problem whatsoever.  But you aren't IFR until you have those elements; a clearance and squawk.

@MichaelHall Not true. Class G is an exception to that rule. You can take of into class G airspace in IMC without a clearance or squawk code, so long as you're following a designated IFR departure procedure. FAR § 91.173 only requires a clearance when operating in controlled airspace, and FAR § 1.1 defines "controlled airspace" as being classes A-E.

@HiddenWindshield, got an example of a SID that is in class G you can steer me to?

@MichaelHall I'm not instrument rated myself yet, so I used the wrong terminology. You can take off in class G IMC using a SID, but you can also use an IFR obstacle avoidance procedure. Sorry for the confusion. At any rate, to answer your question, check out KLOT, the Lewis University Airport in Romeoville, Illinois, just outside Chicago.

@HiddenWindshield, If you are NOT operating under an IFR clearance, (i.e. clearance limit, altitude, squawk code) you are by default operating under VFR.  Entering IMC does not make you IFR.  The VFR rules specify cloud clearance requirements in class G airspace.  If you intentionally enter IMC you are in violation of these rules.  Saying that an IFR clearance is only required in controlled airspace might be a technically true statement, but doesn’t give you free license to fly around IMC in uncontrolled airspace and call it IFR because you are not following instrument flight rules!

Now let’s look at a SID out of KLOT:  The floor of the class E airspace is 700 AGL.  The airfield elevation is 679’ and the SIDs all have you roughly flying runway heading to 1180’, then vectors assigned by ATC.  So are you telling me that when you get your instrument rating you will plan to take off into hard IMC while operating under visual flight rules, climb to 500’ AGL, level off, then cold call ATC to ask for a vector, squawk, and try to file a pop-up flight plan airborne?  Inside a mode C veil and under Class B airspace?!  Good luck with that…

@MichaelHall First, you asked for a class G airport with a SID. I understood that to mean you didn't think class G airports could even have SIDs. So I just found one and posted it. No, it would not be safe to do that from KLOT, but I never claimed it would be. My apologies for misunderstanding. So, try KALX instead. Second, you say that if you are "NOT operating under an IFR clearance ... you are by default operating under VFR". In class G, THIS IS NOT TRUE. If you are following the procedures, then you can be IFR without a clearance. I've posted links to the FARs that explain it.

Let us continue this discussion in chat.

@MichaelHall -- I recently read your comments in the now-frozen chat room https://chat.stackexchange.com/rooms/109270/discussion-between-hiddenwindshield-and-michael-hall .  It happens that a friend of mine is a charter piilot flying King Airs, and a FAA-appointed DPE for everything from gliders to multi-engine and instrument.  His opinion is that flight in IMC in class G airspace is legal, given that the aircraft is appropriately equipped and the pilot has the appropriate rating.  His opinion is that such flight would be considered to be IFR flight, not VFR flight.

@MichaelHall (ctd) -- Also, his opinion is that no clearance would be necessary.  Btw, the topic originally arose when a third person was describing a glider flight in cloud several decades ago that set a state altitude record that still stands today.  No IFR flight plan was filed, but my friend was of the opinion that the flight was legal as long as it was conducted entirely in Class G airspace.

For others reading these comments-- "DPE" = "Designated PIlot Examiner".  DPE's give check rides for ratings.

@HiddenWindshield -- why did you include in your answer the phrase " specifically when they're flying a pre-set departure or arrival procedure from an airport."?  Where in the FARs does it say you must be flying a pre-set departure or arrival procedure from an airport to legally fly IFR in Class G airspace?

VFR on Top is flying VFR while on an IFR flight plan:

What is VFR-On-Top? First things first: it’s ONLY for pilots operating
  under an IFR flight plan.
VFR-On-Top is a request to fly at a VFR altitudes in lieu of an
  assigned altitude. You must ask permission from ATC to operate
  VFR-On-Top.
Why would you want to operate VFR-On-Top? The most common reason is to
  avoid bad weather. You may need to avoid turbulence, ice or some other
  unsavory condition, but the altitude you want is tied up by another
  aircraft.
When the conditions permit (ie VFR conditions) you can ask to fly
  above the clouds or in between layers.
What are the rules governing VFR-On-Top? A VFR-On-Top clearance has
  its limitations. Here are a few you need to follow:
Pilots may not fly below minimum en route IFR altitudes because you
  could crash into the terrain. Minimum en route altitudes exist
  regardless of IFR or VFR weather conditions and weather assigned a
  specific altitude or VFR conditions on top. VFR-on-top is not
  authorized in Class A airspace. Altitudes as prescribed by 14 CFR Part
  91, Section 91.159 must be maintained. Don’t remember 91.159? Here it
  is from Cornell Law School’s website:
CFR 91.159 VFR cruising altitudes or flight level
Any VFR cruising altitude appropriate to the direction of flight
  between the MEA and 18,000 feet MSL may be selected that allows the
  flight to remain in VFR conditions. Any change in altitude must be
  reported to ATC, and pilots must comply with all other IFR reporting
  procedures. The pilot is responsible for traffic avoidance VFR-On-Top
  is NOT a cancellation of IFR flight plan. You must still adhere to ATC
  directions. If you can’t stay VFR you must request an IFR altitude
  from ATC.

From my 2-year experience working on flight plans in ATM software, the answer is no. The flightplan must either be VFR or IFR. If the pilot, plane and route qualifies for both, the pilot may flip a coin before submitting the flight plan, but he/she must choose one rule set.

Not all aircraft are required to be on a flight plan.

@StephenS, correct for VFR, but you need to file a flight plan if you are going to fly under IFR. At least in the US...

@MichaelHall There is no regulation requiring an IFR flight plan entirely within uncontrolled airspace. Doing so in CONUS without running afoul of 91.13 may be difficult, but it is at least theoretically possible—and it is completely normal outside CONUS.

No, it is possible to fly in instrument meteorological conditions in uncontrolled airspace without a flight plan, (this is different than an IFR flight with a terminal point in class G...) but it Is not correct to say that doing so complies with instrument flight rules.

@MichaelHall -- re " it is possible to fly in instrument meteorological conditions in uncontrolled airspace without a flight plan"  -- did you mean legally?  If so, are you meaning to say there is a way to fly legally in IMC but not under IFR?  Interested to know whether this is your position.

@MichaelHall -- re " it is possible to fly in instrument meteorological conditions in uncontrolled airspace without a flight plan"  -- did you mean legally?  If so, are you meaning to say there is a way to fly legally in IMC but not under IFR?  Interested to know whether this is your position.

@quiet flyer, no... in fact my position is directly opposite to this.  I am trying challenge those who believe that way to defend their position.  So far I am seeing declarative statements from individuals who can’t or won’t provide any rational justification.  (And whose profiles don’t provide any clue to their level of expertise...)

Operating in accordance with a set of rules is vastly different than operating in the absence of, or outside of, a given set of rules.  I am simply trying to get some people to acknowledge this simple truth...

It's not possible to simultaneously comply with the IFR and VFR cruising altitude requirements, unless perhaps your aircraft is 500 ft tall.

Those don't apply at lower altitudes.

I can't find any regulation that says "every flight is either VFR or IFR", maybe because it's such a fundamental thing. 14 CFR 1.1 defines IFR and SVFR as differing from VFR, but doesn't actually define VFR itself.
The only explicit statement I can see is 91.101:

This subpart prescribes flight rules governing the operation of
  aircraft within the United States and within 12 nautical miles from
  the coast of the United States.

In other words, 91 Subpart B applies to all aircraft in US airspace. I'd take that to mean that where it just says "do this" then it applies to all flights (e.g. 91.119); if it says "do this under VFR" then it's a VFR-only rule (e.g. 91.159); if it says "do this under IFR" then it's IFR-only (e.g. 91.179).
If you're looking for 'evidence' that VFR/IFR applies to every flight, one place is the FAA flight plan format, which has (had) only three options: VFR, IFR and DVFR. Or the PHAK Glossary, which says "if you can't operate VFR, you must operate IFR":

Visual flight rules (VFR). Flight rules adopted by the FAA governing aircraft flight using visual references. VFR operations
  specify the amount of ceiling and the visibility the pilot must have
  in order to operate according to these rules. When the weather
  conditions are such that the pilot cannot operate according to VFR, he
  or she must use instrument flight rules (IFR)

Or the many sections of the AIM, ATC Orders etc. that talk about the differences between VFR and IFR procedures. Maybe someone else will be able to find something more explicit.


text:
Does a glider pilot need to file an IFR flight plan to legally fly in cloud in class E airspace in the US?
Does a glider pilot need to file an IFR flight plan to legally fly in cloud in class E (controlled) airspace in the US? Or could he or she contact ATC and receive a clearance to enter cloud without actually filing an IFR flight plan?
Assume that the glider pilot has an instrument rating in airplanes (as required by FAR 61.e.3), and the glider has all the equipment required to legally operate in Instrument Meteorological Conditions under Instrument Flight Rules.
Background information-- I know of a case several decades ago where a glider pilot legally thermalled high into a towering cumulus cloud to set a state altitude record under conditions similar to those described above.  The state record would not have been granted if the flight had been conducted in violation of any FARs.  I don't know whether or not an IFR flight plan was actually filed.  Note that FARs 91.619 and 91.153 do contain the phrase "Unless otherwise authorized by ATC" when describing what information must be included on in IFR flight plan, so if an IFR flight plan was filed in this case, it may have looked quite different from a typical one.
Related--
Does a request for a Pop-Up IFR clearance constitute filing a flight plan?
Can you file an open-ended IFR flight plan?
What IFR clearances may a pilot obtain from ATC directly without a pre-filed flight plan with FSS?
What IFR clearances may a pilot obtain from ATC directly without a pre-filed flight plan with FSS?

You can get a pop-up IFR clearance with a block altitude from ATC.

Related: https://aviation.stackexchange.com/questions/25709/does-a-request-for-a-pop-up-ifr-clearance-constitute-filing-a-flight-plan

Thanks for the comments-- seeing as how a request for an IFR clearance from a glider pilot is pretty unusual at least in most parts of the country, I suppose his odds of the request being speedily granted would likely be better if he did either file a written plan before takeoff, or filed in flight with a Flight Service Station?  Maybe he talked his plans over with FSS personnel well in advance of the flight and then filed in flight with them?

Here's the working link for FAR 61.e.3 -- https://www.law.cornell.edu/cfr/text/14/61.3

As answered here, requesting a pop-up IFR from ATC counts as "filing" a flight plan.
It should be noted, as explained here, that airfiling with ATC is a "workload permitting" activity, meaning ATC is not obligated to grant it. They can always refer you to FSS.
Climbing or descending through a cloud layer is one of the generally accepted use cases for pop-up IFR requests, so while ATC might be surprised by a glider asking for it, I can't see why they wouldn't treat it the same as an airplane requesting the same.
The main challenge I see, at least for non-motor gliders wanting to climb, would be phrasing the request correctly to get a clearance they can physically comply with. Maybe "circling within Xnm of present position" to allow for thermaling? It's also possible to request a 3D box if you want to do more complicated maneuvers/work; I'm pretty sure I've seen a question here on that.